OLED: The Thinnest of All

Think thin display and you probably think of an LCD panel. But an up-and-coming display technology that uses organic light-emitting diodes (OLEDs) is actually much thinner than an LCDeven thin enough to be placed on a plastic film.

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Think thin display and you probably think of an LCD panel. But an up-and-coming display technology that uses organic light-emitting diodes (OLEDs) is actually much thinner than an LCDeven thin enough to be placed on a plastic film. Since each subpixel in an OLED is controlled individually, you get an image that's sharper than on a CRT and potentially of better quality than on an LCD. OLEDs are as responsive as CRTs, so you don't get any smearing from slow pixel response times.

The inorganic cousin of the OLED is commonplace: LEDs are the glowing lights on most digital clocks. They emit a great deal of light using relatively little power, last much longer than incandescent bulbs, and are more rugged. An LED emits light from a seam between two layers of different materials, which is fine for small points of light but not for large displays. OLEDs emit light through the layers, so many can be fabricated next to each other for large displays.

High-resolution displays have been slow to emerge, but the Kodak EasyShare LS633 camera has a 2.2-inch OLED panel. This full-color, active-matrix OLED display is clearly superior to an LCD in terms of viewing angle and image quality under low-lighting conditions.

One of the problems with active-matrix OLEDs is that they require two transistors per subpixel. Until recently, only expensive polysilicon backplanes were used for OLED production, because of their greater electron mobility. But researchers have started creating OLED structures on cheaper amorphous silicon substrates. In fact, IDTecha joint venture between Chi Mei Optoelectronics and IBMhas demonstrated a 20-inch full-color OLED panel on an amorphous silicon substrate.

Another problem is the lifetime of the OLED materials. In the display industry, lifetime is measured as the length of time it takes for the display to drop to one-half its initial brightness. Last year, red and green OLED materials had lifetimes of only 10,000 to 15,000 hours, but the big problem was that blue materials had lifetimes of less than 1,000 hours. This meant that the color of the panel would shift rapidly toward yellow as the blue materials dimmed.

Both corporate and academic research efforts have gone a long way toward improving the lifetimes of OLED materials. Red and green materials now last for 20,000 or more hours, though blue materials continue to lag behind. Many companies are making considerable investments in OLED research, and we can expect steady progress on these and other problems that must be resolved before large-scale OLED display production begins.

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